A more loss tolerant rtp payload format for mp3 audio
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A More Loss-Tolerant RTP Payload Format for MP3 Audio. <draft-ietf-avt-rtp-mp3-01.txt>. Ross Finlayson LIVE.COM. Existing RTP payload format for MPEG audio (RFC 2250). Packet boundaries are on MPEG frame boundaries Appropriate for MPEG (1 or 2) layer I or II audio

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A more loss tolerant rtp payload format for mp3 audio

A More Loss-Tolerant RTP Payload Format for MP3 Audio

<draft-ietf-avt-rtp-mp3-01.txt>

Ross Finlayson

LIVE.COM


Existing rtp payload format for mpeg audio rfc 2250
Existing RTP payload format for MPEG audio (RFC 2250)

  • Packet boundaries are on MPEG frame boundaries

  • Appropriate for MPEG (1 or 2) layer I or II audio

    • because their frames are true “application data units” (ADUs) - can be decoded independently

  • Less appropriate for layer III audio (“mp3”)

    • mp3 frames contain a back-pointer to earlier frames

    • ADU boundaries don’t correspond to frame boundaries

    • causes noticeable audio glitches when packets are lost


Illustration

frame

frame

frame

frame

ADU

ADU

ADU

Illustration


Illustration1

frame

frame

frame

frame

ADU

ADU

ADU

Illustration


If a frame gets lost

frame

frame

frame

frame

ADU

ADU

ADU

If a frame gets lost...


If a frame gets lost1

frame

frame

frame

frame

ADU

ADU

ADU

If a frame gets lost...

Data received, but useless

Data lost

Data received, but useless



A new rtp payload format for mp3
A new RTP payload format for MP3

  • Identical to the existing format (RFC 2250), except

    • packets contain complete ADUs - not frames

    • uses a different payload type (dynamic)

  • Data-preserving rearrangement of the original

    • no redundancy added => no increase in bit rate

  • However, implementations now need more knowledge of the internals of MP3 frames


Recent additions to this internet draft
Recent additions to this Internet-Draft

  • How to handle streams with layer I or II frames mixed with layer III

    • probably an uncommon situation

  • ‘Pseudo-code’ for converting between regular “MP3 frames” and “ADU frames”


Notes
Notes

  • For more information - including audio illustrations:

    • http://www.live.com/rtp-mp3/

  • Some multicast MP3 server and receiver tools that (optionally) use this payload format:

    • http://www.live.com/liveCaster/

  • Let’s have some more implementations!


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